This application is based upon and claims the benefit of Japanese Patent Applications No. 2000-242784 filed on Aug. 4, 2000, No. 2000-397994 filed on Dec. 27, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a connecting method and a connecting structure of printed circuit boards.
2. Description of the Related Art
Conventionally, solder is used to connect terminals of a flexible printed circuit board and a rigid printed circuit board. For instance, xe2x80x9cGuidance to High Density Flexible Boardxe2x80x9d (written by Kenji Numakura, printed by Nikkan Kogyo Co. Ltd.) discloses on page 100 a connecting structure of a rigid (hard) printed board and a flexible board due to thermal fusion of solder. According to this structure, as shown in FIG. 1, a land 60a of a rigid printed board 60 is connected with a land 61a of a flexible board 61 by solder 62. The flexible board 61 is adhered to the rigid printed board 60 by adhesive 63.
However, when connection of terminals (lands) of the two boards is achieved by using the solder 62 as shown in FIG. 1, if the amount of solder is excessive, solder bridges are occasionally formed between adjacent two lands. That is, when the lands, which are overlapped with each other after the solder is supplied, is heated and pressurized, the solder is fused to have flowability. At that time, despite very low wettability of an insulating substrate material to solder, if the amount of the solder is excessive, the pressurized solder can flow out from the land to the insulating substrate and to the neighboring land so as to form solder bridges.
The present invention has been made in view of the above problems. An object of the present invention is to provide connecting method and connecting structure of printed circuit boards, capable of preventing formation of solder bridges.
According to a first aspect of the present invention, in a connecting method and a connecting structure of first and second printed circuit boards respectively having first plurality of lands and second plurality of lands, a solder resist portion is formed on the second printed circuit board between neighboring two lands of the second plurality of lands, and the first and second printed circuit boards are overlapped with each other so that the solder resist portion is terminated to have an end portion that is interposed between the first and second printed circuit boards.
Thus, because the solder resist portion is formed between the neighboring two lands of the second plurality of lands, solder bridges can be prevented from being formed between the lands even when surplus solder is extruded onto the surface of the second printed circuit board when the two boards are pressurized. In the structure in which the end portion of the solder resist portion is interposed between the first and second printed circuit boards, the region where the first and second printed circuit boards are adhered to each other can be provided sufficiently, and the solder resist portion causes no decrease in adhesive strength.
According to a second aspect of the present invention, a protective film is formed on a second conductive pattern of a second printed circuit board, and a first printed circuit board is overlapped with the second printed circuit board to form a gap exposing the second conductive pattern, between an edge face of the first printed circuit board and an edge face of the protective film. The gap works as an escape space into which surplus solder can be extruded from the connecting portion between the first and second printed circuit boards when the connecting portion is pressurized.
According to a third aspect of the invention, when a connecting portion of first and second printed circuit boards is pressurized, a pressurizing member is used to apply larger pressure to a middle portion of a first conductive pattern on the first printed circuit board than that to an end portion of the first conductive pattern. The pressurizing member preferably has an edge portion that is tapered, recessed, or stepped. Therefore, surplus solder moves from the middle portion receiving larger pressure toward the end portion receiving smaller pressure, resulting in decreased amount of extruded solder.